1. Field of the Invention
The present invention relates to wide-band direct conversion transmission apparatuses that can be applied to wireless mobile terminals for WLAN, WiMAX, or mobile broadcasting systems, and more particularly, to a wide-band direct conversion transmission apparatus that can be used in a wide band, where the maximum carrier frequency is three times higher than the minimum carrier frequency, and remove in-band harmonic components and out-of-band harmonic components.
2. Description of the Related Art
In general, a direct-conversion transmitter converts a baseband signal into a carrier frequency through a single frequency conversion. The up-conversion transmitter is applied to a CDMA terminal using a frequency of approximately 800 MHz, a PCS terminal using a frequency of approximately 1.9 GHz, or a Wireless LAN (WLAN) using a frequency of approximately 2.4 GHz. A conventional narrow-band direct-conversion transmitter according to the related art has been widely used for those applications.
FIG. 1 is a block diagram illustrating a narrow-band direct conversion transmitting apparatus according to the related art.
Referring to FIG. 1, a narrow-band direct conversion transmitting apparatus according to the related art includes a first low-pass filter 11, a second low-pass filter 12, a first mixer 21, a second mixer 22, a local oscillator 31, a local oscillation generator 32, a driver amplifier 41, a power amplifier 42, and a bandpass filter 43. The first low-pass filter 11 that performs low-pass filtering of an I signal among transmission signals including the I signal and a Q signal having a phase difference of 90°. The second low-pass filter 12 performs low-pass filtering of the Q signal. The first mixer 21 mixes the I signal from the first low-pass filter and a first local oscillation signal LI to generate a first RF signal RF1. The second mixer 22 mixes the Q signal from the second low-pass filter 12 and a second oscillation signal LQ to generate a second RF signal RFQ. The local oscillator 31 generates an oscillation signal. The local oscillation generator 32 generates the first and second local oscillation signals LI and LQ by using the oscillation signal from the local oscillator 31. The driver amplifier 41 amplifies an RF signal obtained by adding the first RF signal RFI from the first mixer 21 and the second RF signal RFQ from the second mixer 22. The power amplifier 42 amplifies the RF signal from the driver amplifier 41. The bandpass filter 43 performs band filtering of the RF signal between the power amplifier 42 and an antenna ANT.
In FIG. 1, the first and second mixers 21 and 22 mix a baseband frequency and the local oscillation frequency from the local oscillator 31 to perform up-conversion.
A signal obtained by the up-conversion generally includes odd harmonic components of the oscillation frequency of the local oscillator 31. The up-converted signal including the harmonics passes through the driver amplifier 41 and the power amplifier 42, such that a signal having an appropriate amplitude to be transmitted is produced.
The output signal of the power amplifier 42 includes desired fundamental frequency signals and various harmonic components. When the output signal passes through the bandpass filter 43, the bandpass filter 43 attenuates the signals out of a desired band, and the in-band signal is transmitted through the antenna ANT.
Therefore, in the narrow-band communication system, shown in FIG. 1, the harmonic components caused by the frequency up-conversion by mixers 21 and 22 do not affect another communication system.
FIG. 2 is a conceptual diagram illustrating a frequency spectrum of an output signal and a filtering function of the bandpass filter in the narrow-band direct conversion transmitting apparatus of FIG. 1.
As described above, in FIG. 2, the harmonic components included at the output of the power amplifier are attenuated by the bandpass filter.
As shown in FIG. 1, when a communication is performed using the narrow-band direct conversion transmitter according to the related art, the harmonic components do not interfere another channel because the harmonic components reside on the outside of the used band. Therefore, the harmonic components of the RF frequency that is generated by the up-conversion mixer do not make any harmful impact on the communication.
However, with the development of the modern information society, there have been an increasing need for effective data transfer and an increasing demand for data and video information. Therefore, a need for an increase in data rate has been also increasing rapidly.
In order to achieve the high data rate, the communication frequency band needs to be increased, and thus a wide-band wireless communication system has attracted much attention.
For example, in a wide communication frequency band where a maximum frequency is three or more times higher than a minimum frequency, odd harmonics of an output transmission signal may fall in the same communication frequency band, and as a result degrade the signal-to-noise ratio (SNR) of an adjacent user and deteriorate the quality of the adjacent user's communication in a different channel but still in the same frequency band.
FIG. 3 is a frequency spectrum diagram illustrating an output signal that is up-converted in a wide-band communication.
As shown in FIG. 3, in a wide-band communication system whose bandwidth is so broad to include the third and higher harmonics of an output frequency, the harmonic components fall within the communication frequency band, and thus interfere other channels in the corresponding frequencies within the same communication band.
In order to avoid the problem in the wide-band communication environment, when the transmitting apparatus, shown in FIG. 1, is set to use a different channel frequency, the center frequency of the bandpass filter needs to be varied according to the change in the channel frequency.
However, the center frequency of an off-chip bandpass filter is usually fixed. Therefore, it is impossible to use the narrow-band transmitting apparatus according to the related art, shown in FIG. 1, for the wide-band communication environment of FIG. 3.